1. Field of the Invention
The present invention generally relates to electronic circuits and, more specifically, to switch-mode power supply circuits using a transformer.
2. Discussion of the Related Art
In switch-mode power supplies which use a transformer, one or several static switches (for example, a free wheel diode) present at the transformer secondary can generate voltage peaks while they are turned off (opened) due to a so-called charge recovery phenomenon. The switch needs to stand such reverse overvoltages. Now, the more the switch stands the reverse voltage, the greater its forward voltage drop, and thus the lower the efficiency of the converter.
It has already been provided to connect, in parallel with a free wheel diode, a capacitive element in series with a resistor. Such a series connection is used to filter overvoltages as the diode is turned off. However, such circuits (generally called “snubbers”) are not adaptable and are set on manufacturing. Further, for circuits intended for high power (from a few watts to a few kilowatts), such protections generate significant losses.
U.S. Pat. No. 5,943,225 discloses a switch-mode converter having a passive element (resistor) in parallel with a capacitive element, said capacitive element being in series with a diode.
DE 3639495 A1 discloses a circuit for reducing switching losses and not for limiting overvoltages. An active element of the circuit needs an external control signal depending on the conduction periods.